Although EGFR-targeted therapy significantly prolongs the survival of NSCLC patients with EGFR kinase domain activating mutations, acquired resistance to EGFR tyrosine kinase inhibitors (TKIs) poses a significant clinical problem. Recent clinical studies demonstrated that an increasing number of these resistant NSCLCs undergo epithelial to mesenchymal transition (EMT); however, the molecular basis of acquired EGFR TKI with an EMT phenotype remains elusive. Consequently, patients with the acquired resistance do not benefit from effective therapies. We have demonstrated that the inhibition of mutant EGFR in NSCLC promotes TGF?1-mediated EMT. In patient specimens, C-X-C chemokine receptor type 7 (CXCR7) is significantly upregulated in acquired EGFR TKI resistant NSCLC cells with an EMT phenotype. Prolonged depletion of CXCR7 with shRNA in the resistant cells not only restores epithelial phenotype but also sensitivity to EGFR TKIs. Our central hypothesis is that CXCR7 is a novel therapeutic target which promotes an EMT phenotype in EGFRmutant NSCLC and provides alternate survival/proliferation pathways when mutated-EGFR is inhibited. The overall objective is to determine the mechanism by which CXCR7 promotes EMT and thus resistance to EGFR TKI and determine whether CXCR7 is a superior drug target for NSCLC therapy. In Aim 1, we will determine the mechanism by which CXCR7 promotes survival of EGFR TKI resistant NSCLC. For this aim, we will investigate if a ligand activation of CXCR7 is required for the engagement of the resistant phenotype. Additionally, we will evaluate therapeutic approaches using in vitro and in vivo models to suppress CXCR7 signaling to specifically target EMT-associated NSCLC cells with EGFR-TKI resistance. In Aim 2, we will determine mechanisms responsible for EMT regulation by CXCR7 in NSCLC. To this end, we will investigate how CXCR7 activates downstream transcription factors to support EMT in EGFR mutant NSCLC by using genomics and proteomics approaches, cell culture and complementary transgenic murine EGFR mutant NSCLC models. In Aim 3, we will determine the therapeutic efficacy of targeting CXCR7 to eliminate EGFR TKI resistant cells with EMT. For this aim, we will investigate if EGFR TKI resistant cells with an EMT phenotype emerge through evolution from drug tolerant cells with increase expression of CXCR7 using PDX models and CXCR7 inhibitors. The results obtained from this proposal will facilitate the discovery of prognostic and therapeutic tools to inhibit CXCR7 expression leading to EGFR TKI resistance, to prevent the induction of EMT upon EGFR inhibition, and to provide a rationale to stratify NSCLC patients who become refractory to EGFR TKI with mesenchymal biomarkers for CXCR7-targeted therapeutics.